1. Field of the Invention
The present invention relates to a projection type display device which uses a liquid crystal display element therein.
2. Description of the Prior Art
Conventionally, as an device for displaying a picture or a screen thereon, there is already known a display device, such as so-called a projection type liquid crystal display device in which an optical image, being formed as changes in optical characteristics depending on a picture signal upon a light bulb being composed of the liquid crystal display element, is irradiated by light emitted from an irradiation means so as to project an enlarged picture on a screen. In a liquid crystal display element of twisted nematic (TN) type as a one example of the representative thereof, two (2) pieces of polarization plates are positioned in front and rear of a liquid crystal cell, which is composed of a pair of transparent substrates having transparent film electrodes formed thereon and the liquid crystal put into between them, in such a manner that the polarization plates are different to each other by an angle of 90.degree. in a polarization direction thereof, thereby displaying an image information through controlling the penetrating amount of the light in the incident light therethrough, by combining a function of rotating the plane of polarization due to electro-optic effect of the liquid crystal and a function of selecting polarization component on the polarization plate. In recent years, on the liquid crystal display element of such a transparent type and/or a reflection type, there has been made great and rapid advances in miniaturization or small-sizing of the element itself, as well as in an improvement of performance or capacity, especially in the power of resolution thereof. Therefore, the display device using such the liquid crystal display element also has been advanced in the small-sizing and for the high performance thereof, and, not only for displaying an image by a video signal and so on, as is used in the conventional art, but also a liquid crystal display device of such a projection type is newly proposed for use as a display device for outputting a picture thereon in a personal computer. For such a sort of display device, in particular, the projection type liquid crystal display device, there are requirements that it is small in size, and that a picture which can be obtained on it is bright even in every corner thereof. However, with the projection type liquid crystal display devices of the conventional art, there are problems that it is still large in the size, and that the performance, including such as brightness of the picture which can be finally obtained thereby is not yet sufficient.
For reduction in size of the display device as a whole, small-sizing of the light bulb, i.e., of the liquid crystal display element itself is effective, however, if the liquid crystal display element is small-sized, there are caused such problems that a ratio of a luminous flux amount upon the liquid crystal display element with respect to a total luminous flux amount emitted by a light source (hereinafter, it is called by "an efficiency in light utilization") comes to be low, since an irradiation area where is irradiated by an irradiating means is small, and that peripheral portion of the picture is dark, etc. Further, the liquid crystal display element can only use a polarization light which is polarized in one direction, therefore, only a half of the light can be utilized from a light source emitting light beam of random polarization.
As a means for obtaining a picture which is bright in every corner thereof, there is already known an integrator optic system which uses two (2) pieces of lens arrays, as is disclosed, for example, in Japanese Patent Laying-Open No. Hei 3-111806 (1991). In this integrator optic system, the light emitted from the light source is divided by a plurality of collecting (or condenser) lenses each having a rectangular opening shape and constructing a first lens array, and the emitted lights of the rectangular opening shape are superposed on an irradiation surface (the liquid crystal display element) so as to build up an image by means of a second lens array which is constructed with collecting (condenser) lenses corresponding to the each collecting lens of rectangular opening shape. With this optic system, it is almost possible to uniform distribution in intensity of the light irradiating the liquid crystal display element.
On a while, as an another optical system in which the random polarization light from the light source is once arranged or lined up in a single polarization direction and then is irradiated upon the liquid crystal display device, there is already known a system, as is disclosed, for example, in Japanese Patent Laying-Open No. Hei 4-63318 (1992), in which the random polarization light emitted from the light source is divided into P polarization light and S polarization light by use of a polarization beam splitter and then they are combined by use of a prism.
However, in order to increase the brightness in the conventional integrator optical system, the lens array(s) must be large in the size thereof, therefore, if trying to make the projection type liquid crystal display device small in the size, the brightness is decreased down. Further, even in the optical system using the polarization light beam splitter, if it is made small in the size, the brightness is also decreased down. As a result of this, it is difficult to realize the small-sizing of the device, as a whole, as well as improving the performance in such as the brightness at the same time. Furthermore, in particular in case of the projection type liquid crystal display device, because the various factors can give an effect on property of the picture in quality, including such as the optical characteristic of a projection lens other than the irradiation means mentioned in the above and the optical characteristic of the liquid crystal display element, etc., it is difficult to obtain a display device of small-sized and of showing a good performance in the picture quality, even if only the irradiation means is improved.